1. Field of the Invention
The present invention relates generally to a silent chain power transmitting device in which a silent chain and sprockets are in mesh with each other for power transmitting operation, and more particularly to such a silent chain power transmitting device which is particularly suitable for use in an automobile for driving a camshaft and an auxiliary device of an engine, a transmission, a transfer box, etc.
2. Description of the Related Art
A conventional silent chain power transmitting device includes a silent chain which is formed by articularly connecting together a number of link plates in an endless fashion. The silent chain is stretched over two sprockets spaced at a predetermined center distance on shafts. While a predetermined tensile load is applied to the silent chain, the chain length is measured for inspection to determine whether or not the measured or inspected silent chain is acceptable in view of a predetermined product standard. The predetermined tensile load is a tensile load so determined as to produce a load tension which is about ⅕ to {fraction (1/10)} of an allowable maximum load tension of the silent chain and which is smaller than a load tension applied to the silent chain during normal use. A chain pitch of the thus obtained (accepted) silent chain is within a dimensional tolerance which is set to deviate from a reference value specified by, for example, a product standard, toward the xe2x80x9cplusxe2x80x9d or positive side of the reference value. This means that the chain pitch has a lower tolerance limit equal to the reference value and an upper tolerance limit greater than the reference value.
On the other hand, the sprockets used in the conventional silent chain power transmitting device have a diameter produced with a dimensional tolerance which is set to deviate from a reference value specified by, for example, the produce standard toward the xe2x80x9cminusxe2x80x9d or negative side of the reference value, Thus, a sprocket pitch (chordal pitch) of the sprocket has a dimensional tolerance which is set to deviate from a reference value specified by the product standard toward the xe2x80x9cminusxe2x80x9d or negative side of the reference value. This means that the sprocket pitch (chordal pitch) of the sprockets has an upper tolerance limit equal to the reference value and a lower tolerance limit less than the reference value.
In the conventional power transmitting device, however, partly because the chain pitch of an actually produced silent chain is likely to become greater than a reference value due to a tolerance set to deviate from the reference value toward the positive side of the reference value, partly because the sprocket pitch (chordal pitch) of actually produced sprockets is likely to become smaller than a reference value due to a tolerance set to deviate from the reference value toward the negative side of the reference value, and partly because the silent chain is subjected to a load tension tending to elongate the chain pitch during use of the silent chain transmitting device, the chain pitch of the silent chain while in use becomes greater than the sprocket pitch (chordal pitch) of the sprockets.
Thus, when a bifurcated tooth-like engagement surface of each link plate of the silent chain starts to mesh with a toothed surface of each of the sprockets, mutual interference or collision occurs between the tooth-like engagement surface of the link plate and the sprocket tooth and mutually causes abnormal wear of the link plates and sprockets. In addition, connecting pins used for articulately connecting the adjacent link plates are subjected to undue loads, tending to shorten the lifetime of the silent chain and the sprockets. Another problem is that, at the onset of meshing engagement between the link plates of the silent chain and the sprockets, impact or collision sound is produced.
With the foregoing drawbacks of the prior art in view, it is an object of the present invention to provide a sprocket for use in a silent chain power transmitting device and a silent chain power transmitting device including such sprocket, which are capable of preventing abnormal wear caused due to interference or collision between the sprocket and a silent chain at the onset of meshing engagement therebetween, thereby improving the durability of the silent chain power transmitting device, and which is also capable of preventing generation of impact sound caused due to the aforesaid interference, thereby lowering the level of operation noise (sliding noise) of the silent chain power transmitting device.
Another object of the present invention is to provide a sprocket for a silent chain power transmitting device and a silent chain power transmitting device including such sprocket, which have an increased number of sprocket teeth engaged in bearing a load applied on a silent chain during power transmitting operation, thereby increasing the durability of the sprocket and the silent chain.
In one aspect the present invention provides a sprocket for use in a silent chain power transmitting device including a silent chain trained around the sprocket so as to provide a tension run and a slack run on opposite sides of the sprocket, wherein the sprocket has a chordal pitch designed to become substantially equal to the chain pitch of the tension run of the silent chain while operating under load.
In another aspect the present invention provides a silent chain power transmitting device comprising a driving sprocket and at least one drive sprocket, and a silent chain stretched over the driving and driven sprockets to provide a tension run and a slack run on opposite sides of the driving sprocket, wherein driving and driven sprockets have a chordal pitch designed to become substantially equal to the chain pitch of the tension run of the silent chain while operating under load.
It is important to note that the foregoing passage reciting that the sprocket or sprockets have xe2x80x9ca chordal pitch designed to become substantially equal to the chain pitch of the tension run of the silent chainxe2x80x9d means that the difference, if any, between the chain pitch of the tension run of the silent chain and the chordal pitch of the sprocket/sprockets is within a dimensional tolerance of the chordal pitch of the sprockets.
When the silent chain power transmitting device is operating, the tension run of the silent chain is forced to move from the driven sprocket to the driving sprocket, and the slack run of the silent chain is returned from the driving sprocket to the driven sprocket.
Since the chordal pitch of the sprocket is designed to become substantially equal to the chain pitch of the tension run of the silent chain while operating under loaded condition, link plates which are articulately connected together to form the silent chain are smoothly guided into sliding contact with teeth of the sprocket without causing objectionable interference or collision. By thus smoothly guiding the link plates relative to the sprocket teeth, it is possible not only to prevent generation of impact or collision sound at the onset of meshing engagement between the chain and the sprockets, but also reduce sliding noise of the silent chain during operation.
At a circumferential portion of the sprocket around which the silent chain is wrapping, a load applied to the silent chain during power transmitting operation is shared or borne by an increased number of successive sprocket teeth spaced at a chordal pitch substantially equal to the chain pitch of the chain tension run. It is, therefore, possible to reduce bending forces or loads acting on the individual sprocket teeth and reduce a load to be shared borne by the sprocket tooth which comes first into meshing engagement with the silent chain.
When the silent chain is operating under load condition, the slack run of the silent chain has a chain pitch smaller than the chordal pitch of the sprocket because the slack run is not stretched or otherwise tensioned and is allowed to become loose due to a cumulative elongation of the tension run gathered at the slack run. However, since the silent chain essentially has high elongation rigidity, the chain pitch of the slack run does not decrease to such an extent that meshing engagement between the silent chain and the sprockets is hindered.